DNA polymerase mutants of T4 are known which strongly influence the frequency of mutations occurring spontaneously or after mutagenic treatments. Both mutator and anti-mutator phenotypes have been discovered for transition base pair substitution mutations. Effects of these polymerases on frameshift mutations have been observed, but never systematically defined. The effects of mutationally altered polymerases on frameshift mutations might be expected to provide genetic clues as to the role of the polymerase in frameshift mechanisms of mutation and would identify mutant alleles of DNA polymerases which could be studied biochemically for altered properties which correlate to frameshift production.